As I mention earlier, this evening's Channel 4 News has a scoop. It will broadcast the only known recording of Guy Burgess, one of the Cambridge spy ring who acted as an agent for the Soviet Union.

It was retrieved from the FBI's archives in Washington DC after a freedom of information request by Stewart Purvis, the former ITN chief executive who is now a visiting professor at City University London, and Jeff Hulbert, a City research fellow.

They waited nine months for the FBI to respond to their request. Then the agency decided to declassify the previously secret material and sent them a copy of the tape.

Purvis says Burgess made the tape in 1951. He evidently wanted to place on record his 1938 meeting with Winston Churchill in which they discussed the Munich agreement. On the tape, Burgess is heard imitating Churchill.

Later that year, Burgess defected to Russia along with fellow spy Donald Maclean. Their disappearance was reported across the world but it wasn't until 1956, when the pair appeared at a press conference in Moscow, that the truth was revealed.

Burgess had been a wartime MI6 intelligence officer and a post-war Foreign Office official. He and Maclean, along with three other former Cambridge university graduates - Anthony Blunt, Kim Philby and John Cairncross - were responsible for passing top secret western intelligence to the Soviets.

Purvis says: "Jeff and I first got interested in Burgess because he was a journalist, at the Times and BBC, who became, among other things, a Russian spy. But we could never find a recording of his voice.

"Then we found out there was a transcript of him in the FBI files and thought, 'if there's a transcript there must be a recording.' So we asked.

"I never expected an FBI envelope to turn up in the post at City but when it did we quickly arranged to be filmed opening the envelope and finding a CD inside. We were delighted to give it to Michael Crick of Channel 4 News first and then to everybody else.

"It shows that even the 'hackacademics' in City's journalism department can help out the research work."